In order to cool electronic devices, multiple fans are often operated in parallel in a fan tray, for example in a so-called rack for servers or in the electrical cabinet of a mobile radio station. The use of multiple fans results in redundancy, i.e. if one of the fans fails, the remaining fan or fans ensure that sufficient cooling air continues to be available.
When one of the fans in a fan tray fails, and the other fans continue to run, air would circulate in short-circuited fashion through the stationary fan. This short-circuit flow must therefore be minimized. Fans having non-return flaps that close automatically when air is flowing in the wrong direction are used for this purpose. U.S. Pat. No. 6,174,232 B1, STOLL et al., shows one example of this; it also describes the problem that the non-return flaps that are used represent an additional flow resistance, and consequently reduce fan performance.